The paper presents elaborated methodology for Saint
Petersburg and its outskirts watersheds parameters ascertainment. The
methodology is based on usage of the digital terrain model, which was
created as result of topographic maps digitalization. Usage of the most
popular global digital terrain models such as SRTM, ASTER GDEM
makes difficulties under the conditions of big cities. The models were
processed on the base of Earth surface radar mapping. Therefore, the
altitude values have distortions due to the radar signal reflection from
roofs, structures, trees, scrubs, and so on. Thus, the global models include
noisy data. The distortions can lead to mistakes at determination of the
watersheds boundaries. Topographic maps digitalization allows elaborating
of digital terrain models without distortions of the altitude values. The
watersheds boundaries ascertainment was carried out by means of the
ArcGIS tool Watershed. In addition, we use for the ascertainment and
determination of the catchments areas structure some tools of Quantum
GIS (QGIS) and SAGA.